CONWIP: closed or semi-open queuing network?

Closed queuing networks (CQNs) are generally used to model CONWIP systems. An implicit assumption in the CQN model is that whenever the work‐in‐process (WIP) level goes below the set value, there is always a job waiting for the released kanban. In other words, there are an infinite number of jobs in the external queue and as a result the kanban never waits in the resource pool. Modelling the CONWIP under this unrealistic assumption tends to underestimate the true cycle time of the kanban (and the job). In this technical note, we represent a CONWIP as a semi‐open queuing network (SOQN) in which the job waits in the external queue until a kanban is available or vice‐versa. Using published data and numerical experiments, we point out a modelling flaw of CONWIP systems in the literature and also show that the SOQN approach is a more appropriate model for representing CONWIP systems.